Forming a mechanical joint between the outer surface of a tube and a bore of a flange or wall member by expanding the tube is well known; however, even the most widely used and commercially successful tube expansion processes have limitations. First, many processes cannot produce a smooth, uniform internal tube wall surface. As a result of repeated cold working, roller-type expansion tools often cause spalling and flaking of the inner surface of the tube. Non-rotating forming tools which are expandable and mount on a mandrel have separate forming segments which typically leave inwardly projecting ridges or fins on the internal tube surface. Post-forming treatments remove these weakly attached fragments (i.e., flakes, rims, ridges, etc.) and clean internal surfaces of any material which could damage the system into which the assembly is subsequently installed.
U.S. Pat. No. 2,357,123 issued to Carl A. Maxwell offers approaches to forming a uniformly smooth finish on the internal surface of the tube. Maxwell first proposes to expand the internal tube surface with a split ball expander head comprising a plurality of radially displaceable segments with joints between adjacent segments formed at an angle skewed with respect to the direction of movement of the tool during forming. Tools having such oblique joints or openings between forming segments tend to peel thin strips of material from the tube because they dig into the tube surface during the expansion operation. Maxwell describes a second tool having a pair of longitudinally spaced, expandable, split rings. The design precludes constructing the split rings of hardened tool steel or similar relatively hard, inflexible material because the ring workpiece contact surfaces must flex during expansion. The need to form the split rings from a soft, flexible material limits the wear life of the forming elements and restricts the force which can be transmitted to the tube surfaces during the expansion operation. In addition, it is necessary to prevent rotation of the split rings because, if left free to rotate, they can align their respective openings during the forming operation.
Conventional expansion tools and processes do not uniformly expand tubes from one assembly to the next. Nominal variations in tube diameter and wall thickness dimensions preclude consistent tube deformation by tools which have a predetermined size or mechanical limit to the tool's expanded dimension. Consequently, mechanical joints between tubes and bores formed by expansion of the tube into the bore are often weak due to insufficient expansion, or they are prone to failure as a result of overstressing the tube or wall structures. Maxwell addresses this problem by measuring the pressure required to draw the forming tool through the tube. If this pressure is found to be below a predetermined value, Maxwell adjusts the mechanical stops on the forming tool and repeats the forming process. Not only is this approach inefficient, it also requires double working of the tube wall which can form surface defects and undesirable residual stresses.
U.S. Pat. No. 4,262,518 issued to Todd D. Creger et al. offers another approach to creating a smooth inner tube wall surface; however, Creger et al. retains the disadvantages associated with tools which expand to a predetermined size. Creger's tool includes a plurality of forming pieces circumferentially distributed around and loosely connected to a conical mandrel which has a firm support or stop at its larger end. As the mandrel is drawn through a tube to be expanded, friction drives the forming pieces toward the stop on the mandrel. When the forming pieces abut against the stop, the gaps between them are very small and skewed with respect to the longitudinal axis of the tube, thus the forming surface leaves few distortions on the inner tube wall. Since Creger's tool must expand to a predetermined size, it cannot provide the force necessary to over-expand tube to meet nominal variations in tube diameter and thickness or the size of the bore into which the tube is to expand. In an effort to properly handle joints which require under-expansion, Creger's device limits the hydraulic force applied to draw the expander through the tube. Reducing the hydraulic force prevents the forming pieces from abutting the mandrel stop and closing the gaps between the forming surfaces. This lack of closure leaves a rough surface on the inside surface of the tube.
U.S. Pat. No. 4,597,282 issued to Franciscus Hogenhout offers a different approach to creating a smooth surface on the inner surface of a cold-worked hole; however, Hogenhout retains the disadvantages associated with tools which expand to a predetermined size. Hogenhout has a tool for cold working a hole which includes a plurality of workpiece contacting surfaces circumferentially distributed around a solid mandrel. With the mandrel removed, the tool may be collapsed and inserted into a hole to be worked. The mandrel is inserted into the tool during the cold working process to expand the segments radially outward to a predetermined size. The segments have a first surface and a second surface which is circumferentially offset from the first surface. Gaps between the first workpiece surfaces create radially inwardly projecting fins in the hole which the second workpiece surfaces flatten. The tool provides for a smooth inner wall surface; however, the tool is severely limited in that it must expand to a predetermined size resulting in the same limitations as those set forth in the description of Creger's device above. In addition, the large gaps which must exist between the workpieces to permit the tool to collapse can adversely affect the (circular) cross-sectional shape of the tube once expanded.
The present invention is directed to overcoming one or more of the problems set forth above.